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Abstract

Background

While high protein diets have been shown to improve satiety and retention of lean
body mass (LBM), this study was designed to determine effects of a protein-enriched
meal replacement (MR) on weight loss and LBM retention by comparison to an isocaloric
carbohydrate-enriched MR within customized diet plans utilizing MR to achieve high
protein or standard protein intakes.

Methods

Single blind, placebo-controlled, randomized outpatient weight loss trial in 100 obese
men and women comparing two isocaloric meal plans utilizing a standard MR to which
was added supplementary protein or carbohydrate powder. MR was used twice daily (one
meal, one snack). One additional meal was included in the meal plan designed to achieve
individualized protein intakes of either 1) 2.2 g protein/kg of LBM per day [high
protein diet (HP)] or 2) 1.1 g protein/kg LBM/day standard protein diet (SP). LBM
was determined using bioelectrical impedance analysis (BIA). Body weight, body composition,
and lipid profiles were measured at baseline and 12 weeks.

Results

Eighty-five subjects completed the study. Both HP and SP MR were well tolerated, with
no adverse effects. There were no differences in weight loss at 12 weeks (-4.19 ±
0.5 kg for HP group and -3.72 ± 0.7 kg for SP group, p > 0.1). Subjects in the HP
group lost significantly more fat weight than the SP group (HP = -1.65 ± 0.63 kg;
SP = -0.64 ± 0.79 kg, P = 0.05) as estimated by BIA. There were no significant differences
in lipids nor fasting blood glucose between groups, but within the HP group a significant
decrease in cholesterol and LDL cholesterol was noted at 12 weeks. This was not seen
in the SP group.

Conclusion

Higher protein MR within a higher protein diet resulted in similar overall weight
loss as the standard protein MR plan over 12 weeks. However, there was significantly
more fat loss in the HP group but no significant difference in lean body mass. In
this trial, subject compliance with both the standard and protein-enriched MR strategy
for weight loss may have obscured any effect of increased protein on weight loss demonstrated
in prior weight loss studies using whole food diets.

Background

Meal replacement shakes represent an important strategy in combating the worldwide
epidemic of obesity due to their simplicity and convenience [1]. Meal replacement shakes have been studied extensively for both medical and public
health efforts to combat obesity [2-4].

A number of studies have suggested that protein is the most important macronutrient
mediating satiety and leads to increased weight loss with retention of lean body mass.
Single meals with increased protein to carbohydrate ratios have also been shown to
increase satiety and decrease food intake [5,6], resulting in both improved weight loss and improved maintenance of weight loss [7-9]. Meal replacement simplifies the weight loss regimens by replacing one or two meals
a day with a product of defined nutrient and calorie content. MR leads to increased
weight losses over twelve weeks compared to simply restricting favorite food intakes,
and weight losses have been maintained for up to five years using MR [10]

An increase in dietary protein content has been proposed to be effective for body
weight regulation through effects on satiety, thermogenesis and substrate partitioning.
Protein has specific effects on satiety hormones, including PYY 3–36 [11]. When protein replaces carbohydrate within a low-fat diet, reduced insulinemic and
glycemic responses have been observed resulting in increased fat oxidation [12]

The present study was designed to test the hypothesis that simply increasing the protein
content of a meal replacement (MR) within a high protein diet without the knowledge
of the participant would result in increased weight loss and improved retention of
lean body mass in the absence of a resistance exercise program by comparison to standard
MR within a standard protein diet. To test the hypothesis, a soy and whey protein
powder was used to enrich a standard MR shake in one arm compared to a carbohydrate
"placebo" powder added to the same MR shake in the other arm. This novel approach
has not been tested previously to our knowledge. To minimize variations based on body
composition, the diets were also adjusted so that each subject was instructed to follow
a diet which provided either 2.2 gm/kg lean body mass protein in the high protein
(HP) group or 1.1 gm protein/kg lean body mass in the standard protein (SP) group.
Patients received dietary instruction at baseline, and met with the dietitian at weeks
2, 4 and 8 to assess general compliance and to provide additional supplies of the
MR products. Therefore, this study examines the effectiveness of protein enrichment
of MR in a realistic outpatient setting on weight loss and retention of lean body
mass.

Methods

Subjects were recruited by public advertisement. Subjects over 30 years of age with
a body mass index (BMI) between 27 to 40 kg/m2, and in good health by history, physical examination, and basic laboratory screening
(complete blood count, serum chemistries, liver panel, and lipid panel) were selected
for study. Subjects with type 2 diabetes or glucose intolerance were excluded as were
individuals who regularly drank more than one alcoholic beverage daily,

One hundred men and women who met the selection criteria were randomly assigned to
either the HP or SP treatment. This was a single-blinded study. The protein powder
jars were labeled as either A or B, depending on their protein content. Subjects were
randomized in a 1:1 manner to either HP or SP diet for 12 weeks using a computerized
random proportion model. Diet plans were individualized per subject. Caloric intake
to achieve weight loss was based on a 500 Kcal deficit of the participants' estimated
resting metabolic rate as determined by body composition analysis by bioelectrical
impedance.

Participants in the HP group received a diet plan that provided 2.2 grams of protein
per kg of LBM while the diet for the SP group provided 1.1 grams of protein per kg
of LBM. The meal energy macronutrient composition in the HP group was approximately
30% protein, 30% fat, and 40% carbohydrate. The macronutrient composition in the SP
diet was approximately 15% protein, 30% fat, and 55% carbohydrate. Both groups received
the same isocaloric MR (Formula 1, Herbalife Intl., Los Angeles) with either a protein
supplement for the HP group (Performance Protein Powder, Herbalife Intl., Los Angeles)
or with a similar tasting carbohydrate placebo for SP group. Two MR and two meals
were eaten daily.

Instructions were provided for preparation of the MR and subjects were advised to
consume one MR as a meal and the other as snack. All subjects were given individualized
menu plans that incorporated the two MR (one meal and one snack) and included two
all-food meals. All participants met individually with a registered dietitian at baseline
for dietary instruction, and at 2, 4, and 8 weeks to assess compliance.

Participants were weighed and protein powder meal replacement products were dispensed
at each visit to ensure compliance. Subjects were given general advice for increasing
their activity level with a goal of 30 minutes of aerobic exercise per day, but no
heavy resistance exercise.

Body weight and composition

Subjects were weighed at each visit (Detecto-Medic; Deteco-Scales; Brooklyn, NY) while
wearing no shoes and after an overnight fast. Height was measured with a stadiometer
(Detecto-Medic; Deteco-Scales; Brooklyn, NY) at week 0. BMI was calculated as weight
(kg)/height squared (m). Body composition was determined by bioelectrical impedance
analysis (BIA) (310e Bioimpedance analyzer; Biodynamics; Seattle, WA) and was performed
at 0 and 12 weeks.

Biochemistry

Statistical analysis

Weight loss was the primary outcome and the data were analyzed according to intention
to treat allocation utilizing SAS version 9 (Cary, North Carolina) in the Department
of Biostatistics.

Patient characteristics and baseline measurements of the two study groups were compared
using t-test (for numerical variables) or Chi-square test (for categorical variables)
to evaluate quality of the randomization.

Standard t-tests were used to compare weight losses between the two arms. In addition,
to assess weight loss within each treatment arm, paired t-tests were conducted comparing
baseline and 12 week weight for each subject. All data except baseline characteristics
are presented as means +/- standard error of the mean (SEM). A univariate analysis
of variance was used to assess differences between treatment and outcome variables.
Since the distributions of change in fat weight and percentage change in fat weight
were not normal, signed rank test was used for testing change from baseline within
each group. The Wilcoxon rank sum test was used for comparing the change between the
two groups. Multivariate analysis was performed to compare the difference between
the two diet groups using general linear model. Square root transformation was applied
before the multivariate analysis was performed.

Results

100 obese men and women were randomly assigned to either a HP or SP MR diet plan.
Fifteen subjects withdrew from the study within the first week after randomization
due to noncompliance with the meal plan (6 in the HP group and 9 in the SP group).
All other subjects completed the 12-week study. Subject characteristics in the two
treatment arms at baseline were not significantly different (Table 1). Mean age was 49.4 ± 1.1 years. Mean BMI at baseline was 33.8 ± 0.53 for HP group
and 32.6 ± 0.58 kg/m2 for SP group.

Weight loss

Subjects were weighed at baseline, and at 2, 4, 8 and 12 weeks. Baseline body weight
was not significantly different between these two groups. Both groups lost significant
amount of weight at 12 weeks (-4.19 ± 0.5 kg for HP group and -3.72 ± 0.7 kg for SP
group, p < 0.0001 for both groups). (Figure 1) After controlling for baseline weight, gender, and time period, there was no significant
difference between the two treatment groups. For both dietary groups, BMI was significantly
lower at 12 weeks (HP = -1.50 ± 0.58; SP = -1.13 ± 0.24). There were no significant
differences in BMI changes between the two dietary groups (Table 2).

Cholesterol, HDL, LDL, triglyceride, and glucose

At 12 weeks, there were significant reductions in cholesterol and LDL levels (mg/dL)
for the HP group (cholesterol -13.2 ± 5.3, p < 0.05; LDL -7.47 ± 3.38, p < 0.05) but
not for the SP group (cholesterol -7.02 ± 4.3 p > 0.05; -9.17 + 5.65, p > 0.05). The
difference between the two groups was not significant. There were no significant changes
from baseline, nor between dietary groups in serum HDL and triglyceride levels. Similarly,
fasting blood glucose levels did not change significantly from baseline for either
group at 12 weeks. (Table 3)

Discussion

Protein-enriched meal replacements within a higher protein diet resulted in no greater
overall weight loss than the standard protein MR plan over 12 weeks. In this trial,
the amounts of weight lost were typical for meal replacement studies done previously
[10]. However, the expected effects on increased weight loss resulting from a high protein
diet were not seen in this study. There are two possible reasons for the observed
similarities in overall weight loss. First, the subjects in the SP group may have
eaten foods outside their recommended meal plans which increased protein intake enough
to compensate for the difference in protein contents of the MR. Second, the use of
MR may have been the major influence on the weight loss by simplifying their weight
loss efforts so that the power of the MR intervention may have obscured the difference
between the weight loss of subjects using protein-enriched MR shakes by comparison
to standard MR [13]. The purpose of the study was to test the real world impact of simply enriching MR
with more protein. Based on our results, it appears that compliance is a much more
important factor in the MR regimen than protein content.

Protein enrichment of MR did appear to lead to increased retention of lean body mass
based on bioelectrical impedance analysis. In this study, greater retention of lean
body mass was suggested by the observation of increased fat loss at similar weight
losses. Fat loss is determined by subtracting the lean body mass determined based
on body water content from the total body weight at baseline compared to 12 weeks.
The fat loss was significant both between groups and in individuals over time. Loss
of lean mass was not different since the variability in fat free mass between subjects
increased the variability of this measurement, reducing our power to see any difference.
The observation we made at 12 weeks using bioelectrical impedance will require confirmation
in longer-term studies where changes in body composition are more marked and in which
additional methods for determining body composition are used. A recent meta-analysis
of 87 short-term diet studies where protein and carbohydrate content was varied found
that a protein intake of greater than 1.05 g/kg of actual body weight was associated
with 0.6 kg additional fat-free mass retention compared with diets with protein intakes
≤1.05 g/kg [14]. Both meal plans in this study had protein greater than this cut point and the effects
seen may be blunted by the relatively high protein in the SP group.

In future studies, it may also be desirable to combine protein enrichment of MR with
resistance exercise to demonstrate significant differences in the retention of lean
body mass during weight loss due to protein enrichment of MR. Evans and co-workers
have shown that healthy free-living elderly men and women accommodate to the Recommended
Dietary Allowance (RDA) for protein of 0.8 grams/kilogram/day with a continued decrease
in urinary nitrogen excretion and reduced muscle mass. Increased dietary protein intake
(up to 1.6 g protein/kg/day) may also enhance the hypertrophic response to resistance
exercise that would enhance weight loss maintenance [15].

Conclusion

In summary, both the HP and SP diets resulted in the expected weight loss typical
of an MR diet plan at 12 weeks. Both diets were well tolerated, sustainable, and did
not result in any adverse effects. While typical results for outpatient trials of
MR were observed in both arms, greater compliance with the MR diet plan may be necessary
to obtain an improved sense of the contribution of protein enrichment of MR to lean
body mass retention during weight loss. Finally, future studies may be more successful
if they include a comparison of standard MR and protein-enriched of MR weight reduction
regimens combined with heavy resistance exercise to maintain or increase lean body
mass.

Competing interests

The authors declare that they have no competing interests.

Authors' contributions

All authors read and approved the final manuscript. LT participated in the conduct
of the study, the analysis of the data, and drafted the manuscript. SC participated
in the conduct of the study. KH participated in the conduct of the study. EY participated
in the conduct of the study. CLC participated in the study design and statistical
analysis. GT participated in the study coordination. HW participated in the statistical
analysis. RE participated in the statistical analysis. ZL conceived of the study,
participated in its design and coordination, and drafted the manuscript.